Dielectric ceramics has been widely used as dielectric resonators, MIC-use dielectric substrates and waveguides in high frequency ranges of microwaves, millimeter waves and the like. For such uses, the ceramics is required to have the following characteristics;
1) A high dielectric constant .di-elect cons..sub.r to meet the needs for miniaturization of devices as a wavelength in a dielectric material having a dielectric constant .di-elect cons..sub.r is shortened to 1/.di-elect cons..sub.r.sup.1/2 times that in vacuum;
2) A low dielectric loss at high frequencies, that is, high Q value; and,
3) A small change .DELTA.f/.DELTA.T (=.tau..sub.f) in resonance frequency f with respect to temperature T, that is, low dependence and high stability of the dielectric constant with respect to temperature.
Oxide ceramic materials, such as BaO--TiO.sub.2 based materials, BaO--REO--TiO.sub.2 based materials (REO stands for a rare-earth metal oxide) and MgTiO.sub.3 --CaTiO.sub.3 based materials, have been known as this kind of dielectric ceramics (refer to Japanese Patent Publication JP-61-10806A, Japanese Patent Publication JP-63-100058A and Japanese Patent Publication JP-60-19603A, for example).
Although the BaO--TiO.sub.2 based materials have a dielectric constant .di-elect cons..sub.r in the range of 37 to 40 and a high Q value of about 40000, it has been impossible to obtain a single-phase material having a very nearly zero temperature dependence .tau..sub.f of resonance frequency. Furthermore, the dielectric constant of such a single-phase material changes significantly with a change in composition, and also the dielectric constant thereof also changes significantly with temperature. Therefore, it has been difficult to control the composition to obtain a low temperature coefficient .tau..sub.f of resonance frequency in the state where the dielectric constant is maintained high and where the dielectric loss is maintained low.
In addition, regarding the BaO--REO--TiO.sub.2 -based materials, BaO--Nd.sub.2 O.sub.3 --TiO.sub.2 -based and BaO--Sm.sub.2 O.sub.3 --TiO.sub.2 -based materials have been known, and it has been possible to obtain materials having a very high dielectric constant .di-elect cons..sub.r in the range of 40 to 60, and a temperature coefficient .tau..sub.f of resonance frequency of 0 (zero). However, these materials have a small Q value of 5000 or less.
Furthermore, in the case of the MgTiO.sub.3 --CaTiO.sub.3 -based materials, it has been possible to obtain materials having a high Q value of 30000, and a temperature coefficient .tau..sub.f of resonance frequency of 0 (zero). However, these materials have a small dielectric constant in the range of 16 to 25.
As described above, any of the above-mentioned materials dose not sufficiently satisfy the above-mentioned three characteristics required for high-frequency dielectric materials.